Microdevice and its production method

ABSTRACT

A micro device including an insulating substrate having a recess formed on a surface, and a beam-like silicon structure on the front surface of the insulating substrate surrounding the recess. The beam-like structure includes at least one functional section, and the functional section has a supporting section bonded to the insulating substrate and at least one cantilever integral with the supporting section and extending across the recess. The micro device also has an electrically conductive film electrically connected to the supporting section, on the surface of the recess at least directly under a cantilever. The electrically conductive film prevents the surface of the recess from being positively charged in the dry etching process. Thus, an etching gas having a positive charge is not subjected to electrical repulsion from the recess and does not impinge on the back surface of the silicon substrate, and therefore erosion of the cantilever does not occur. As a result, since the beam-like structure is formed with high accuracy in shape and dimensions, the micro device has improved reliability and an improved degree of freedom in design.

TECHNICAL FIELD

The invention relates to a micro device used in inertial force sensor,optical switch or the like, and particularly to a micro devicecomprising an insulating substrate and a beam-like structure made ofsilicon formed on the insulating substrate, and a method ofmanufacturing the same.

BACKGROUND ART

Recently it has been made possible to etch silicon as deep as 100 μm bymeans of reactive ion etching (RIE) technology using inductively coupledplasma (ICP) as the activation energy source (hereinafter referred to asICP-RIE process). This technique is viewed as a promising new techniquefor making silicon structures of high aspect ratios with a sufficientlyhigh etching rate, in the field of device development by amicromachining. In the past, the wet process using an alkali solutionwas predominant as the process of deep etching of silicon substrates.But it is difficult to make a desired structure by the wet process,because the direction of etching depends on the crystal orientation ofsilicon in the wet process. In contrast, the ICP-RIE process is notsubject to anisotropy of etching because it is a dry process. Thus theICP-RIE process has such an advantage over the wet process that farhigher degree of freedom in designing the configuration of structure canbe achieved than in the case of the wet process.

When machining by dry etching a silicon substrate whereon a mask filmhas been formed in a desired pattern by photolithography or the like,however, there occurs such a problem that a wider area (exposed througha wider aperture) is etched at a higher rate than a narrower area. Thisis caused by micro loading effect, which is a well-known phenomenon inthe field of semiconductor manufacturing processes. This phenomenon hassuch an adverse effect as described below on the micro devices whichfall in the scope of the present invention, namely micro devicescomprising an insulating substrate and a beam-like structure made ofsilicon formed on the insulating substrate.

FIG. 15 and FIG. 16 show the structure of an inertial force sensor as anexample of basic structure of a micro device 100 of the prior art. FIG.15 is a schematic plan view and FIG. 16 is a sectional view taken alonglines XVI-XVI′ of FIG. 15. The inertial force sensor 100 comprises aninsulating substrate 101 having a recess formed in the surface thereof,and a beam-like structure 104 made of silicon so as to interpose therecess on the surface of the insulating substrate 101. The beam-likestructure 104 further comprises two electrodes 105, 105. The electrode105 comprises a supporting section 106 and a plurality of cantilevers107. The cantilevers 107 are arranged to oppose each other via a minuteclearance.

FIGS. 17A-17G are sectional views schematically showing themanufacturing process of the inertial force sensor shown in FIG. 15 ofthe prior art. A similar manufacturing process has been proposed, forexample, by Z. Xiao et al. in Proc. of Transducers ′99, pp. 1518-1521,and S. Kobayashi et al. in Proc. of Transducers ′99, pp. 910-913.

A silicon substrate 103 is provided in the step of FIG. 17A, and a glasssubstrate 101 is provided in the step of FIG. 17B. A mask film 108 isformed on the surface of the glass substrate 101 by photolithography inthe step of FIG. 17C, and a recess 102 is formed by etching the surfaceof the glass substrate 101 to a depth in a range from severalmicrometers to several tens of micrometers with a dilute solution ofhydrofluoric acid in the step of FIG. 17D. In the step of FIG. 17E, thesilicon substrate 103 is bonded onto the surface of the glass substrate101 by anodic bonding. In the step of FIG. 17F, a mask film 109 having apattern that corresponds to the planar configuration of the beam-likestructure 104 shown in FIG. 15 is formed by photolithography. In thestep of FIG. 17G, the silicon substrate 103 is etched through by theICP-RIE process, to form a cantilever 107. Then the resist remaining onthe surface of the silicon substrate is removed.

The step of FIG. 17G involves a problem. The mask film 109 in the stepof FIG. 17F generally has both of wide apertures and narrow apertures.Consequently, when a dry etching process such as the ICP-RIE process isapplied to the silicon substrate 103 that has the mask film 109, thesilicon substrate is etched at a higher rate in a portion exposedthrough the wider aperture than in a portion exposed through thenarrower aperture due to the micro loading effect. As a result, thewider portion is etched through earlier than the narrower portion in thesilicon substrate 103. At this time, etching gas enters into theclearance between the recess 102 of the glass substrate 101 and the backsurface of the silicon substrate 103 through the hole which has beenetched out in the silicon substrate 103 earlier. The etching gas whichhas entered erodes the back surface of the silicon substrate 103 tillthe narrowest portion is completely etched out. Thus the side wall ofthe supporting section 106 and the bottom surface or the side wall ofthe cantilever 107 are eroded. As a result, dimensions of the beam-likestructure 104 deviate significantly from the designed values, making itimpossible to obtain the target characteristics of the device.

Erosion of the supporting section and the cantilever due to the microloading effect can be restricted by making the sizes of all aperturescomparable when designing. However, this approach imposes severelimitation to the freedom of designing the device structure. Even whenthe dimensions of apertures are set to be the same in design, it isdifficult to completely prevent the erosion of the supporting sectionand the cantilever in the actual process. This is because it is a commonpractice to apply over-etching to some extent in order to etch throughreliably.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a microdevice which has a beam-like structure that provides a sufficient degreeof freedom in the design of the device structure by restricting theerosion of the supporting section and the cantilever due to the microloading effect, and a method of manufacturing the same.

The present inventors have completed the present invention by findingthat the problem described above can be solved by a micro device havingan electrically conductive film which is formed on a recessed surface atleast in a portion right under a cantilever of an insulating substrateand is electrically connected with a supporting section.

Specifically, the micro device of the present invention comprises aninsulating substrate having a recess formed on the surface thereof, anda beam-like structure made of silicon formed on the front surface of theinsulating substrate to surround the recess, wherein the beam-likestructure comprises at least one functional section and the functionalsection has a supporting section bonded onto the insulating substrateand at least one cantilever formed integrally with the supportingsection while extending across the recess. The micro device also has anelectrically conductive film formed on the surface of the recess atleast in a portion right under a cantilever.

The micro device of the present invention has the following features.

Erosion of the supporting section made of silicon and the cantilever iscaused, as described above, by the etching gas which enters into theclearance between the recess of the insulating substrate and the backsurface of the silicon substrate which has been etched through earlierduring the dry etching process. The silicon substrate is etched in sucha mechanism of dry etching as activated ions having positive charge areaccelerated by a negative bias formed right above the silicon substratethereby to collide with the silicon substrate with a sufficient energy.In the case of the ICP-RIE process, the activated etching gas is usuallysulfur fluoride ion (SFx⁺). The ion turns into silicon fluoride (SiFx)through reaction with silicon, and is discharged to the outside. Thenegative bias is formed above the silicon substrate by applying a highfrequency to a substrate holder that also serves as a cathode whereonthe silicon substrate is placed.

Therefore, erosion of the back surface of the silicon substrate isconsidered to occur as the SFx⁺ that has entered the clearance betweenthe back surface of the silicon substrate and the recess of theinsulating substrate is repulsed by the surface of the insulatingsubstrate and collides with the back surface of the silicon substrate.Repulsion of the SFx⁺ on the surface of the insulating substrate may becaused also by electrical repulsion force as well as kinematicscattering. The electrical repulsion force will be described below withreference to FIG. 13 and FIG. 14.

FIG. 13 is a schematic sectional view showing a silicon substrate 45bonded onto the surface of an insulating substrate 41, which has arecess, so as to surround the recess 42, in a state of the siliconsubstrate 45 being dry-etched. The silicon substrate 45 has a mask film50 formed on the surface thereof for the purpose of forming a functionalsection. The silicon substrate 45 is formed into a supporting section 46and a plurality of cantilevers 47 through dry etching.

During the dry etching process, the surface of the recess 42 of theinsulating substrate 41 is charged with positive charge 52 by theetching gas which impinges thereon a number of times, for example, SFx⁺51. The surface of the recess 42 charged with the positive chargerepulses the SFx⁺ 51 which comes next. The repulsed SFx⁺ 51 changes thedirection of the movement thereof before reaching the recess 42 andinstead impinges on the back surface of the silicon substrate 45. Alsoit may be that the SFx⁺ 51 which is bound to hit the insulatingsubstrate 41 at right angles is distracted from the trajectory by therecess 42 that is positively charged, and impinges on the side wall ofthe supporting section 46.

Therefore, in order to restrict the erosion of the back surface of thesilicon substrate 45 or the supporting section 46, it is effective toprevent the surface of the recess 42 of the insulating substrate 41 frombeing positively charged.

According to the present invention, as shown in FIG. 14, theelectrically conductive film 43 is formed on the surface of the recess42 of the insulating substrate 41, and the surface of the recess 42 ofthe insulating substrate 41 is prevented from being positively chargedby electrically connecting the electrically conductive film 43 and thesupporting section 46. In this case, when the etching gas collides withthe electrically conductive film 43, charge of the etching gas isdischarged through the supporting section 46, thereby deactivating theetching gas. Since the silicon substrate 45 has the same potential asthe substrate holder which is held at a negative potential during dryetching, charge of the etching gas is neutralized upon collision withthe electrically conductive film 43, so that deactivation isaccelerated.

While it suffices to form the electrically conductive film used in thepresent invention on the surface of the recess at least in a portionright under the cantilever, it is preferable to use the electricallyconductive film formed over the entire surface of the recessed, whichmakes it possible to prevent the entire surface of the recess from beingcharged thereby restricting the erosion of the back surface of thesilicon substrate more effectively.

The inertial force sensor of the present invention comprises aninsulating substrate having a recess formed on the surface thereof, anda beam-like structure made of silicon formed on the front surface of theinsulating substrate so as to interpose the recess, wherein thebeam-like structure comprises a movable electrode and a fixed electrode,with the movable electrode and the fixed electrode each having asupporting section bonded onto the insulating substrate and acomb-shaped electrode comprising a plurality of cantilever electrodesformed integrally with the supporting section while extending across therecess. The cantilevers of the movable electrode and the fixed electrodeare arranged to oppose each other via a minute clearance. In theinertial force sensor having such a constitution, an electricallyconductive film which is electrically connected with the supportingsection is formed on the surface of the recess at least in a portionright under the cantilever.

The inertial force sensor of the present invention has the electricallyconductive film which is formed on the surface of the recess at least ina portion right under the cantilever, for the purpose of preventing thesurface from being charged, and is electrically connected with thesupporting section. Thus when the cantilever is formed by dry etching,the supporting section and the cantilever are not subject to erosionbecause the etching gas having positive charge loses the charge uponcollision with the electrically conductive film and is neutralized. As aresult, since there occurs no variation in the distance between thecantilevers that constitute the movable electrode and the comb-shapedelectrode of the fixed electrode, such an inertial force sensor can beprovided as the deterioration of sensitivity and variation in thecharacteristic are suppressed.

The micro device of the present invention comprises an insulatingsubstrate having a recess formed on the surface thereof, a beam-likestructure made of silicon formed on the front surface of the insulatingsubstrate so as to interpose the recess, an optical fiber holder whichis fastened on the beam-like structure and holds a plurality of opticalfibers disposed at a predetermined distance, and electromagneticattraction means fastened to oppose the back surface of the insulatingsubstrate and the beam-like structure. The beam-like structure comprisesa supporting section which has an aperture and cantilevers formedintegrally with the supporting section. The supporting section is bondedonto the insulating substrate and has a fixed mirror provided at one endof the inner wall of the aperture, while the cantilever is formed tooverhang from the other end of the inner wall of the aperture, with amovable mirror being provided to erect on the surface at the tip of thecantilever to oppose the fixed mirror. A magnetic film that reacts withthe electromagnetic attraction means is formed on the back surface ofthe cantilever, so that the electromagnetic attraction means attractsthe back surface of the tip of the cantilever onto the recess of theinsulating substrate, via the magnetic film, thereby switching themirror, that reflects the light incident from the optical fiber, fromthe movable mirror to the fixed mirror, thus switching the optical pathand allowing the application as an optical switch.

The optical switch described above has an electrically conductive filmwhich is formed on the surface of a recessed in a portion at least rightunder the cantilever of the insulating substrate and is electricallyconnected with the supporting section. As a result, when the siliconsubstrate is processed to form the cantilever by reactive etching, theetching gas having positive charge collides with the electricallyconductive film and loses the charge thereby to be deactivated, andtherefore the etching gas does not erode the back surface of thecantilever. Thus since the cantilever having a high accuracy of theprofile is formed, such an optical switch can be provided as thedeterioration of response characteristic during switching of the opticalpath and variation in the characteristics are suppressed.

The method of manufacturing the micro device of the present invention,which comprises the insulating substrate having the recess formed on thesurface thereof and the beam-like structure made of silicon formed onthe front surface of the insulating substrate so as to interpose therecess, wherein the beam-like structure comprises at least onefunctional section and the functional section has the supporting sectionbonded onto the insulating substrate and at least one cantilever formedintegrally with the supporting section while extending across therecess, comprises a step of forming the electrically conductive film onthe surface of the recess at least in a portion right under thecantilever of the insulating substrate and extending the electricallyconductive film over the surface around the recess thereby to establishelectrical continuity with a supporting section; a step of forming afirst mask film which corresponds to the configuration of the supportingsection on the surface of the silicon substrate; a step of forming thesupporting section by etching the surface of the silicon substratewhereon the first mask film has been formed; a step of bonding thesilicon substrate which has the supporting section and the insulatingsubstrate which has the electrically conductive film so that thesurfaces thereof oppose each other; a step of forming a second mask filmwhich corresponds to the configuration of the cantilever on the backsurface of the silicon substrate that has been bonded; and a step ofetching the back surface of the silicon substrate having the second maskfilm formed thereon to penetrate through the silicon substrate by dryetching, thereby to form the cantilever of a desired pattern whichextends across the recess.

According to the manufacturing method of the present invention, theelectrically conductive film is formed on the surface of the recess ofthe insulating substrate for the purpose of preventing the surface frombeing charged. At this time, a part of the electrically conductive filmis extended over the surface around the recess thereby to form anelectrical lead to the supporting section. As the electricallyconductive film is electrically connected to the supporting section, theelectrically conductive film is kept at the same potential as thesubstrate holder which is electrically connected to the supportingsection, and is subjected to a negative bias. Thus when etching the backsurface of the silicon substrate, which has the second mask film formedthereon, thereby to penetrate through the silicon substrate by dryetching, the etching gas having positive charge collides with theelectrically conductive film and loses the charge thereby to bedeactivated, and therefore the etching gas does not erode the backsurface of the cantilever. As a result, since the side wall of thesupporting section and the bottom surface or side wall of the cantileverare not eroded, it is not necessary to design the apertures of the maskfilm to have similar dimensions. Thus the present invention can providethe manufacturing method of the micro device having the high accuracybeam-like structure made of silicon and a high degree of freedom ofdesign.

For the dry etching process to form the cantilever, it is desirable toemploy the ICP-RIE process, which makes it possible to form thebeam-like structure mad of silicon having a high aspect ratio in ashorter period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome more apparent from the following description of the preferredembodiments thereof made with reference to the accompanying drawings,throughout which like parts are designated by like reference numeralsand which:

FIG. 1 is an exploded perspective view schematically showing the overallstructure of the acceleration sensor according to the first embodimentof the present invention;

FIG. 2 is a plan view showing the structure of the acceleration sensoraccording to the first embodiment of the present invention, focusing onthe beam-like structure;

FIG. 3 is a plan view showing the structure of the acceleration sensoraccording to the first embodiment of the present invention, focusing onthe lower glass substrate and the electrically conductive film;

FIG. 4 is a sectional view taken along lines IV-IV′ of FIG. 2 showingthe structure of the acceleration sensor according to the firstembodiment of the present invention;

FIG. 5 is an exploded perspective view schematically showing the overallstructure of the optical switch according to the second embodiment ofthe present invention;

FIG. 6 is a sectional view taken along lines VI-VI′ of FIG. 5 showingthe structure of the optical switch according to the second embodimentof the present invention;

FIGS. 7A-7H are sectional views (part 1) schematically showingmanufacturing processes according to the third embodiment of the presentinvention, for manufacturing the acceleration sensor of the firstembodiment;

FIGS. 8I-8R are sectional views (part 2) schematically showing processesfor manufacturing the acceleration sensor of the first embodiment;

FIGS. 9A-9C show the structure of the beam-like structure bonded ontothe insulating substrate according to an embodiment of the method ofmanufacturing the micro device of the present invention, FIG. 9A being asectional view schematically showing the structure after etching by theICP-RIE process, FIG. 9B being an enlarged perspective view of an impactprotection stopper after etching, and FIG. 9C being an enlargedperspective view of the cantilever after etching;

FIGS. 10A-10C show the structure of the beam-like structure bonded ontothe insulating substrate in a first comparative example, FIG. 10A beinga sectional view schematically showing the structure after etching bythe ICP-RIE process, FIG. 10B being an enlarged perspective view showingthe structure of the impact protection stopper after etching, and FIG.10C being an enlarged perspective view showing the structure of thecantilever after etching;

FIGS. 11A-11C show the structure of the beam-like structure bonded ontothe insulating substrate in a second comparative example, FIG. 11A beinga sectional view schematically showing the structure after etching bythe ICP-RIE process, FIG. 11B being an enlarged perspective view showingthe structure of the impact protection stopper after etching, and FIG.11C being an enlarged perspective view showing the structure of thecantilever after etching;

FIGS. 12A-12C show the structure of the beam-like structure bonded ontothe insulating substrate in a third comparative example, FIG. 12A beinga sectional view schematically showing the structure after etching bythe ICP-RIE process, FIG. 12B being an enlarged perspective view showingthe structure of the impact protection stopper after etching, and FIG.12C being an enlarged perspective view showing the structure of thecantilever after etching;

FIG. 13 is a drawing (part 1) showing the operating principle of thepresent invention;

FIG. 14 is a drawing (part 2) showing the operating principle of thepresent invention;

FIG. 15 shows the structure of the micro device of the prior art, in aplan view of the beam-like structure having the basic structure;

FIG. 16 shows the structure of the micro device of the prior art, in asectional view taken along lines XVI-XVI′ of FIG. 15; and

FIGS. 17A-17G are sectional views schematically showing the method ofmanufacturing the micro device shown in FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

As an example of the micro device of the present invention, anapplication thereof to acceleration sensor will be described below. FIG.1 is an exploded perspective view of the acceleration sensor, FIG. 2 isa plan view of the acceleration sensor focusing on the beam-likestructure made of silicon, FIG. 3 is a plan view of the accelerationsensor focusing on the lower glass substrate and the electricallyconductive film, and FIG. 4 is a sectional view of the accelerationsensor taken along lines IV-IV′.

As shown in the perspective view of FIG. 1, the acceleration sensor 1comprises a beam-like structure 21 made of silicon, a lower glasssubstrate 2 having a recess 3 formed in the surface thereof, and anupper glass substrate 6 having a recess 7 formed in the surface thereof.The acceleration sensor 1 has such a structure as the beam-likestructure 21 is bonded so as to interpose between the lower glasssubstrate 2 and the upper glass substrate 6 so that the recess 3 and therecess 7 oppose each other, with the inside of the sensor beinghermetically sealed. The upper glass substrate 6 has electrode leads 9,10 which penetrate through the substrate for connecting to outsidecircuits, while the electrode leads 9, 10 make contact with thebeam-like structure 21 in electrical continuity.

As shown in FIG. 1, the beam-like structure 21 comprises two functionalsections, namely a movable electrode 22 and fixed electrodes 23, 23, anda sealed section 24. Further as shown in FIG. 2, the movable electrode22 comprises a comb-shaped electrode 25 which consists of a plurality ofcantilevers 26 extending across the recess 3 and a base portion 27,impact protection stoppers 28, 28 which are provided on both sides ofthe comb-shaped electrode 25 and restrict the vertical displacement ofthe comb-shaped electrode 25, two beams 29, 29 which are linked with theimpact protection stoppers 28, 28 and support the comb-shaped electrode25 and the impact protection stoppers 28, 28 in space, and supportingsections 30, 30 which are bonded onto the lower glass substrate 2 so asto support the beams 29, 29. The impact protection stopper 28 also hasan effect of protecting the beams 29, 29 and the comb-shaped electrode25 from being damaged by strong impact. The movable electrode 22 isintegrally formed.

The fixed electrodes 23, 23 each comprises a comb-shaped electrode 31which consists of a plurality of cantilevers 31A arranged to oppose theplurality of cantilevers 26, that extend across the recess 3, of thecomb-shaped electrode 25 of the movable electrode 22 via a minuteclearance, and a supporting section 32 which supports the comb-shapedelectrode 31 and is bonded onto the lower glass substrate 2. The fixedelectrodes 23, 23 are each integrally formed.

As shown in FIG. 3, the electrically conductive film 4 is formed overthe entire surface of the recess 3 of the lower glass substrate 2. Apart of the electrically conductive film 4 extends over the surfacearound the recess 3, and constitutes an electrical lead 5 forestablishing electrical continuity with the supporting section 23 of themovable electrode 22. The supporting section 23 is bonded right abovethe electrical lead 5 as shown in FIG. 1.

While the electrical lead 5 is located right under the supportingsection 23 of the movable electrode 22 in this embodiment, similareffect can be achieved also by forming the electrical lead right underthe supporting section 32 of the fixed electrode 23.

As shown in FIG. 4, a metal film 8 is formed in the recess 7 of theupper glass substrate 6. The metal film 8 is provided for the purposeof, when carrying out anode bonding of the beam-like structure 21 andthe upper glass substrate 6, preventing the movable electrode 22 whichhas been formed earlier from sticking onto the upper glass substrate 6.

The acceleration sensor 1 of the present invention senses horizontalacceleration in the plane of the beam-like structure 21 made of silicon.The comb-shaped electrodes 25, 31 constituted from a plurality ofcantilevers contribute to the maximization of facing area of theopposing electrodes, that is proportional to a change in theelectrostatic capacitance, within a limited area of the sensor.

The electrically conductive film 4 which is electrically connected tothe supporting section 23 prevents the surface of the recess from beingpositively charged when forming the comb-shaped electrodes 25, 31 by theICP-RIE process. Specifically, when the etching gas having positivecharge collides with the electrically conductive film 4 during dryetching, the positive charge is discharged through the supportingsection 23, and the etching gas is deactivated by the negative potentialof the supporting section 23. Since the etching gas having positivecharge is subjected to electrical repulsion of the recess 3 and does notimpinge on the back surface of the silicon substrate, the etching gasdoes not erode the side walls of the comb-shaped electrodes 25, 31, theimpact protection stoppers 28, 28 and the supporting sections 30, 32. Asa result, the acceleration sensor of the present invention is lesslikely to suffer from deterioration in the sensitivity and variation inthe performance among sensors, since the clearance between the pluralityof cantilevers of the comb-shaped electrode is formed with a highaccuracy and weight of the movable electrode and the fixed electrode canbe controlled to a desired value.

While the electrically conductive film may be made of variouselectrically conductive materials such as chromium, aluminum, nickel,tantalum, platinum and gold which are metals that can be deposited byvapor deposition, chromium which deposits well on the glass substrate ispreferable. Thickness of the electrically conductive film is in a rangefrom 10 nm to 1 μm, and preferably from 200 nm to 500 nm. Whenthethickness is below 10 nm, sufficient durability cannot be ensuredduring the reactive etching process, and a film larger than 1 μm inthicknessrequires longer time to form.

While the insulating substrate may be made of any insulating material aslong as the material can be processed into a desired shape, glasssubstrate is preferable.

Embodiment 2

As an example of the micro device of the present invention, anapplication to optical switch will be described below. FIG. 5 is anexploded perspective view schematically showing the structure of anoptical switch 60, and FIG. 6 is a sectional view of the optical switch60 taken along lines VI-VI′. The optical switch 60 of the presentinvention comprises an insulating substrate 61 having a recess 62 formedon the surface thereof, a beam-like structure 65 made of silicon formedon the front surface of the insulating substrate 61 so as to interposethe recess 62, an optical fiber holder 73 which is fastened on thebeam-like structure 65 and holds a plurality of optical fibers 74disposed at a predetermined distance, and electromagnetic attractionmeans 75 which is not shown and is fastened to oppose the back surfaceof the insulating substrate 61 and the beam-like structure 65.

The beam-like structure 65 comprising a supporting section 66 which hasan aperture 67 and cantilever 68 constitutes a functional section. Thebeam-like structure 65 further has a fixed mirror 72 provided at one endof the inner wall of the aperture 67, while the cantilever 68 is formedto overhang from the other end of the inner wall of the aperture 67,with a movable mirror 71 being provided to erect on the surface at thetip 69 of the cantilever 68 to oppose the fixed mirror 72. A magneticfilm 70 that reacts with the electromagnetic attraction means 75 so asto be attracted thereby is formed on the back surface of the cantilever68. An electrically conductive film 63 is formed on the surface of therecess 62 of the insulating substrate in a portion located at leastright under the cantilever 68, and a part of the electrically conductivefilm 63 extending over the surface around the recess 62 forms anelectrical lead 64 that is in electrical continuity with the supportingsection 66 of the silicon substrate. The electromagnetic attractionmeans 75 comprises a first permanent magnet 76 which fixed on an uppersupport substrate 80, a second permanent magnet 77 which is fastenedonto the back surface of the insulating substrate 61 so as to interposethe cantilever 68, and an electromagnet 79 which is fastened to surroundthe permanent magnet 77 and has a coil 78 wound thereon.

When there is no current flowing in the electromagnet 79, the cantilever68 is separated from the insulating substrate 61 by a predetermineddistance, and the tip 69 is held at a position of making contact withthe upper support substrate 80, so that light from the optical fiber 74is reflected by the movable mirror 71. When a current flows in theelectromagnet 79 so as to magnetize in the same direction as the secondpermanent magnet 77, the magnetic film 70 is attracted so that thecantilever 68 is locked in such a state as the tip 69 is attracted tothe insulating substrate 61. At this time, the mirror that reflects thelight incident from the optical fiber 74 is switched from the movablemirror 71 to the fixed mirror 72. Thus the light incident from theoptical fiber 74 is reflected by the fixed mirror 72, thereby switchingthe optical path.

According to this embodiment, since the electrically conductive film 63that is in electrical continuity with the supporting section 66 of thesilicon substrate 65 is formed on the surface of the recess 62 of theinsulating substrate 61 right under the cantilever 68, the surface ofthe recess is prevented from being charged when forming the cantilever68 by the reactive dry etching process. Thus the etching gas havingpositive charge is not subjected to electrical repulsion of the recess62 and does not impinge on the back surface of the silicon substrate,and therefore erosion of the cantilever 68 does not occur. As a result,since the beam-like structure is formed with high accuracy in the shape,dimensions and weight, deterioration in the response characteristic canbe suppressed when switching the optical path and high reliability canbe provided to the optical switch.

Embodiment 3

The manufacturing method of the present invention will be describedbelow by taking the acceleration sensor as an example of the microdevice. FIGS. 7A-7H, 8I-8R are sectional views schematically showing theprocess of manufacturing the acceleration sensor of the firstembodiment.

In the steps of FIGS. 7A to 7D, the silicon substrate is processed onthe surface thereof to form the supporting section of the beam-likestructure, while the electrically conductive film is formed on the lowerglass substrate in the steps of FIGS. 7E to 7H. The silicon substrateand the lower glass substrate are bonded together and the siliconsubstrate is processed to form the comb-shaped electrode of thebeam-like structure in the steps of FIGS. 8I to 8K. The upper glasssubstrate that has been processed in the step of FIG. 8P is bonded ontothe beam-like structure in the step of FIG. 8Q, and the electrodeleading portion is formed on the upper glass substrate in the step ofFIG. 8R, thereby completing the acceleration sensor having the structurecorresponding to the sectional view of FIG. 4. The process will now bedescribed in detail below for each step.

In the step of FIG. 7A, a silicon substrate 20 (400 μm thick) having athermal oxidation film 33 which is 1 μm in thickness formed on thesurface thereof is prepared. In the step of FIG. 7B, the thermaloxidation film 33 of the silicon substrate 20 is removed using bufferedhydrofluoric acid. In the step of FIG. 7C, the first mask film 34 isformed from a resist in accordance to the shape of the supportingsection by photolithography on the surface of the silicon substrate 20.In the step of FIG. 7D, the silicon substrate 20 having the first maskfilm 34 is etched to a depth of 250 μm by the ICP-RIE process. Then theresist remaining on the surface is removed, and the supporting section32, the sealing portion 24 and the impact protection stopper 28 areformed.

In the step of FIG. 7E, the lower glass substrate 2 (400 μm thick) isprepared. In the step of FIG. 7F, the mask film 12 is formed from aresist for the formation of recess by photolithography on the surface ofthe lower glass substrate 2. In the step of FIG. 7G, the surface of thelower glass substrate 2 is etched to a depth of 20 μm using a 10%aqueous solution of hydrofluoric acid, thereby to form the recess 3. Inthe step of FIG. 7H, a Cr film is formed over the entire surface of therecess 3 and to partially extend over the surface around the recess 3 byphotolithography, thereby to form the electrically conductive film 4made of Cr. The electrically conductive film 4 which extends over a partof the portion around the recess 3 form the electrical lead 5 thatelectrically connects with the silicon substrate 20.

In the step of FIG. 8I, the surface of the lower glass substrate 2 andthe surface of the silicon substrate 20 are bonded together by anodebonding process. At this time, the electrically conductive film 4 andthe silicon substrate 20 are connected with each other by the electricallead 5. In the step of FIG. 8J, a second mask film 35 is formed from aresist by the photolithography processing on the surface of the siliconsubstrate 20. Then a thermal oxidation film mask 33 is formed byelectron cyclotron resonance reactive ion etching process (hereinafterreferred to as ECR-RIE process). In the step of FIG. 8K, back surface ofthe silicon substrate 20 is etched to a depth of at least 150 μm by theICP-RIE process using the second mask film 35 and the thermal oxidationfilm 33 as the masks. This results in the formation of the comb-shapedelectrodes 25, 31 penetrating the silicon substrate 20. Only thecantilever 26 of the comb-shaped electrode 25 is shown in the drawing.Then the thermal oxidation film 33 that remains on the back surface ofthe silicon substrate 20 is removed by the ECR-RIE process. Depth 150 μmof etching is determined by subtracting the depth of etching 250 μm inthe step of FIG. 7D from the thickness 400 μm of the silicon substrate20.

In the step of FIG. 8I, the upper glass substrate 6 (400 μm thick) isprepared. The mask film 13 is formed from a resist for the formation ofthe recess 7 by photolithography on the surface of the upper glasssubstrate 6. In the step of FIG. 8N, the surface is etched to a depth of20 μm using a 10% aqueous solution of hydrofluoric acid, thereby to formthe recess 7. In the step of FIG. 8O, a Cr film is formed on the surfaceof the recess 7 by the photolithography process, thereby to form theanti-sticking film 8 made of Cr. In the step of FIG. 8P, the upper glasssubstrate 6 is sand blasted to provide the electrode-leading portion 10constituted from through hole.

In the step of FIG. 8Q, the back surface of the silicon substrate 20 andthe front surface of the upper glass substrate 6 are bonded together byanode bonding. Then an electrode film 11 made of Pt is formed on theelectrode leading portion 10 in the step of FIG. 8R, thereby completingthe acceleration sensor 1.

While the dry etching process well known in the prior art may beemployed in the manufacturing method of the present invention, it ispreferable to employ the ICP-RIE process which is capable of forming abeam-like structure having a high aspect ratio in a shorter period oftime.

Now experiments for verifying the effects of the manufacturing method ofthe invention will be described below.

The first embodiment, in which the electrically conductive film 4 wasformed from Cr on the surface of the recess 3 of the lower glasssubstrate 2, the electrical lead 5 was formed right under the supportingsection 32 and the beam-like structure was formed by etching the siliconsubstrate by the ICP-RIE process, corresponds to the third embodiment.In the first comparative example, the beam-like structure was formed bya process similar to the third embodiment, except that the electricallyconductive film 4 was not formed on the surface of the recess 3 of thelower glass substrate 2. In the second comparative example, thebeam-like structure was formed by a process similar to the thirdembodiment, except that the thermal oxidation film 33 was left to remainas the protective film on the bottom surface of the impact protectionstopper 28. In the third comparative example, the beam-like structurewas formed by a process similar to the third embodiment, except that theelectrically conductive film 4 was formed only on the surface of therecess 3 of the lower glass substrate 2, without providing theelectrical lead 5.

The mask patterns used in the four experiments have apertures of widthsin a range from 5 μm to 50 μm. The minimum width 5 μm is the distancebetween the cantilevers that constitute the movable electrode and thecomb-shaped electrode of the fixed electrode. These values are common asthe design value for such a micro device. The rate of etching silicon bythe ICP-RIE process was estimated prior to the experiments, with theresult of 2.0 μm/min. in a portion with aperture width of 5 μm and 3.3μm/min. in a portion with aperture width of 50 μm. This difference iscaused by the micro loading effect. Thus the etching time is calculatedto be 75 minutes by dividing 150 μm by 2.0 μm/min. for a portion underan aperture 5 μm in width, and 45 minutes by dividing 150 μm by 3.3μm/min. for a portion under an aperture 5 μm in width. This means thatthe back surface of the silicon substrate is exposed to the etching gasfor 30 minutes obtained by 75 min. minus 45 min. till the substrate iscompletely etched through.

Results of these experiments will be described below. FIGS. 9A-9C to12A-12C show the results of the embodiment and the comparative examples1 to 3, respectively. FIGS. 9A-12A, FIGS. 9B-12B and FIGS. 9C-12C areschematic sectional views showing the structures of the insulatingsubstrate and the silicon substrate after being etched by the ICP-RIEprocess, FIGS. 9B-12B being an enlarged perspective view of the impactprotection stopper after etching, and FIGS. 9C-12C being an enlargedperspective view of the cantilever after etching. FIGS. 9B-12B and FIGS.9C-12C are schematic diagrams prepared from photographs of the backsurface of the silicon substrate taken with a scanning electronmicroscope (SEM). The alternate dot and dash line in the drawingindicates the original profile of the impact protection stopper 28.

In the case of the embodiment, bottom surface of the impact protectionstopper 28 was not eroded. The cantilever 26 maintained substantiallyvertical side wall. In the case of the first comparative example, bottomsurface of the impact protection stopper 28 was severely eroded, showingover etching of 40 μm over the design value. The cantilever 26 waseroded on the side wall resulting in narrowed tip. In the case of thesecond comparative example, bottom surface of the impact protectionstopper 28 was eroded in a conical shape with the apex located on thethermal oxidation film 33. The thermal oxidation film 33 is used as amask in the ICP-RIE process, and is etched at a rate as slow as aboutone hundredth that of silicon. Therefore, silicon in a portion coveredby the thermal oxidation film 33 is protected, but the more distant fromthe covered portion, the more silicon is eroded. The cantilever 26 waseroded on the side wall resulting in narrowed tip, similarly to thefirst comparative example. In the case of the third comparative example,the impact protection stopper 28 and the cantilever 26 were both erodedsimilarly to the first comparative example.

The results described above show that the effects of the presentinvention cannot be achieved simply by forming the electricallyconductive film 4 on the surface of the recess 3 of the lower glasssubstrate as in the third comparative example, in which case the etchinggas cannot be prevented from eroding the back surface of the siliconsubstrate 20. When the bottom surface of the impact protection stopper28 is eroded as in the case of the first comparative example and thethird comparative example, function of the impact protection stoppercannot be achieved because the space between the lower glass substrate 2and the silicon substrate 20 is enlarged. Also because erosion of theimpact protection stopper 28 results in mass loss of the movableelectrode, sensitivity of the sensor deteriorates including the case ofthe second comparative example. Also because erosion of the cantileverthat occurred in the first to third comparative examples leads toincreased distance between the cantilevers, deterioration in sensitivityof the sensor and variation in the characteristics among devices arecaused.

As described above, as the micro device of the present invention has theelectrically conductive film, which is electrically connected with thesupporting section, being formed on the surface of the recess of theinsulating substrate at least in a portion right under the cantilever,the insulating substrate can be prevented from being charged during dryetching. As a result, since erosion of the cantilever and the supportingsection can be prevented, the beam-like structure made of silicon can beformed with high accuracy in the profile and dimensions. Thus highreliability and high degree of freedom in design are ensured.

Since the micro device of the present invention has the electricallyconductive film formed over the entire surface of the recess, the entiresurface of the recess can be prevented from being charged thus making itpossible to suppress the erosion of the cantilever and the supportingsection more effectively.

The inertial force sensor of the present invention has the electricallyconductive film, which is electrically connected with the supportingsection, being formed on the surface of the recess at least in a portionright under the cantilever that constitutes the comb-shaped electrode,the comb-shaped electrode and the supporting section can be preventedfrom being charged during dry etching. As a result, the comb-shapedelectrode and the supporting section can be formed with high accuracy inthe profile and dimensions, deterioration in sensitivity of the sensorand variation in the characteristics among devices can be suppressed.Thus an acceleration sensor or an angular velocity sensor having highreliability can be provided.

The method of manufacturing the micro device of the present inventionincludes a process of forming the electrically conductive film whichprevents the insulating substrate from being charged during dry etching,and therefore the micro device having beam-like structure made ofsilicon with high accuracy in the profile and dimensions can bemanufactured. Also because the manufacturing process is not affected bythe micro loading effect, degree of freedom in design of the microdevice having the beam-like structure can be improved significantly.

The manufacturing method of the present invention is capable of formingthe beam-like structure of a high aspect ratio in a shorter period oftime by employing the ICP-RIE process for dry etching.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedhere that various changes and modifications will be apparent to thoseskilled in the art. Therefore, unless such changes and modificationsotherwise depart from the spirit and scope of the present invention,they should be constructed as being included therein.

What is claimed is:
 1. A micro device comprising: an insulatingsubstrate having a recess at a surface of the insulating substrate; abeam-like silicon structure on the surface of the insulating substrate,surrounding the recess, the beam-like structure comprising at least onefunctional section having a supporting section bonded to the insulatingsubstrate and at least one cantilever integral with the supportingsection and extending across the recess; and an electrically conductivefilm electrically connected to the supporting section and on a surfaceof the recess, at least in a portion directly under the cantilever. 2.The micro device according to claim 1, wherein the electricallyconductive film covers all of the recess.
 3. An inertial force sensorcomprising: an insulating substrate having a recess at a surface of theinsulating substrate; a beam-like silicon structure on the surface ofthe insulating substrate, interposed in the recess, said beam-likestructure comprising a movable electrode and a fixed electrode, with themovable electrode and the fixed electrode each having a supportingsection bonded to the insulating substrate, and a comb-shaped electrodecomprising a plurality of cantilever electrodes, integral with thesupporting section and extending across the recess, the movableelectrode and the fixed electrode opposing each other with a clearancebetween them; and an electrically conductive film electrically connectedto the supporting section and on a surface of the recess, at leastdirectly under the cantilever.
 4. A method of producing a micro devicecomprising an insulating substrate having a recess at a surface and abeam-like silicon structure on the insulating substrate, interposed inthe recess, said beam-like structure including at least one functionalsection having the supporting section bonded to the insulating substrateand at least one cantilever integral with the supporting section andextending across the recess, the method comprising: forming anelectrically conductive film on the surface of the recess at leastdirectly under the cantilever and extending around the recess toestablish electrical continuity with a supporting section; forming afirst mask film, which corresponds to the supporting section, on asurface of a silicon substrate; forming the supporting section byetching the silicon substrate on which the first mask film has beenformed; bonding the silicon substrate that has the supporting section tothe insulating substrate which has the electrically conductive film sothat surfaces of the silicon and insulating substrates oppose eachother; forming a second mask film which corresponds to the configurationof the cantilever on a back surface of the silicon substrate; and dryetching the back surface of the silicon substrate and penetrates throughthe silicon substrate, to form a cantilever which extends across therecess.
 5. The method according to claim 4, including dry etching byreactive ion etching, using an inductively coupled plasma (ICP-RIE), toform the cantilever.